Filters are commonly utilized in the processing of electrical signals. For example, in communications applications, such as microwave applications, it is desirable to filter out the smallest possible passband and thereby enable dividing a fixed frequency spectrum into the largest possible number of bands.
Historically, filters have fallen into three broad categories. First, lumped element filters utilize separately fabricated air wound inductors and parallel plate capacitors, wired together to form a filter circuit. These conventional components are relatively small compared to the wave length, and thus provide a compact filter. However, the use of separate elements has proved to be difficult to manufacture, resulting in large circuit to circuit variations. The second conventional filter structure utilizes three-dimensional distributed element components. These physical elements are sizeable compared to the wavelength. Coupled bars or rods are used to form transmission line networks which are arranged as a filter circuit. Ordinarily, the length of the bars or rods is one quarter or one half of the wavelength at the center frequency of the filter. Accordingly, the bars or rods can become quite sizeable, often being several inches long, resulting in filters over a foot in length. Third, printed distributed element filters have been used. Generally, they comprise a single layer of metal traces printed on an insulating substrate, with a ground plane on the back of the substrate. The traces are arranged as transmission line networks to make a filter. Again, the size of these filters can become quite large. These filters also suffer from various responses at multiples of the center frequency.
The parallel-coupled microstrip bandpass filter is a commonly used filter and has been widely utilized in the last few decades because of its planar structure, simple design and implementation, and wide bandwidth range. In high frequency circuit sections, such as the RF stage of transmitter and receiver circuits for communication systems, microstrip bandpass filters are often used to attenuate harmonics radiation caused by the nonlinearity in amplifier circuits. Microstrip filters are also commonly employed to eliminate undesired signal waves such as interfering waves, sidebands, etc. from the desired signal waves. When utilizing a common antenna for both the transmitter and the receiver circuits, microstrip filters may also separate the transmitter frequency band and the receiver frequency band.
FIG. 1 is an illustration of a traditional prior art bandpass filter. With reference to FIG. 1, a multi-resonator bandpass filter 100 comprises a plurality of quarter wavelength (λ/4) sequentially coupled microstrip lines 111-115. Generally, prior art bandpass filters utilize straight microstrip lines; however, the bandpass filter may also utilize bent microstrip lines commonly referred to as hairpin transmission lines or hairpin resonators. FIG. 2 is a graph of the frequency response of the prior art bandpass filter of FIG. 1 having a passband of 10.24 GHz to 11.78 GHz. With reference to FIG. 2, the return loss 202 and insertion loss 203 characteristics of the prior art bandpass filter are shown where the measured minimum loss in the passband was approximately −9.871 dB at 10.24 GHz and −9.713 dB at 11.78 GHz. To reduce spurious passbands at the harmonics of the center frequency, the specific frequency range of 21.28 GHz to 23.12 GHz should be attenuated. Additionally, to reduce any passbands resulting from spurious or undesired signals, the frequency range of 15.96 GHz to 17.34 GHz should also be attenuated. The measured minimum loss at these frequency ranges in the traditional prior art bandpass filter was approximately −39.795 dB at 15.96 GHz and −42.586 dB at 17.34 GHz and −21.046 dB at 21.28 GHz and −28.690 dB at 23.12 GHz.
As illustrated in FIG. 2, the traditional parallel-coupled microstrip bandpass filter, however, possesses spurious passbands at the harmonics of the designed center frequency (fo). This greatly limits the use of the parallel-coupled microstrip bandpass filters in broadband systems operating over a frequency bandwidth including the second and third harmonics of the designed center frequency of a filter. Since modern communication systems utilize wider bandwidth and filters are essential components within these systems, there exists a need in the art to overcome this problem.
Further prior art methods and apparatuses have attempted to address these problems with typical parallel-coupled microstrip bandpass filters. Several prior art methods include providing different electrical path lengths for the even and odd modes to suppress the second harmonic passband, utilizing a uniplanar compact photonic-bandgap structure to reject both the second and third harmonic passbands, and utilizing wiggly-line bandpass filters. These prior art techniques, however, require a complex circuit design and/or alter the physical size of the filter to pass desired signals without producing significant distortion or to sufficiently attenuate interfering signals outside the passband.
Techniques for directly realizing a bandpass filter having ideal filter characteristics, based on a clear design procedure, are not known in the prior art, and it is thus common practice to construct filters empirically by mixture of various known techniques. For example, bandpass filters for communication applications are generally realized and constructed as filter circuits having the desired passband/stopband characteristics by connecting series or parallel resonant circuits employing various circuit elements in a plurality of stages. In many cases, filter circuit blocks are constructed by unbalanced distributed constant transmission lines such as coupled microstrip lines or patch resonators, because they provide good electrical characteristics for high frequency circuits, and are small in size as circuit elements.
A need exists in the art for compact, reliable, and efficient microstrip filters capable of suppressing the second and third harmonic passbands. Accordingly, there is a need for a method and apparatus for a novel microstrip bandpass resonator that would overcome the deficiencies of the prior art. Therefore, an embodiment of the present subject matter provides a microstrip filter comprising a first microstrip resonator operatively connected to a first feed point, a second microstrip resonator operatively connected to a second feed point, and a third microstrip resonator operatively connected to the first or second resonator, wherein said third resonator is a half wavelength (½λ) resonator. The third resonator may further comprise a plurality of resonators wherein the position thereof with respect to the first or second resonators being a function of a predetermined rejected frequency range.
Another embodiment of the present subject matter provides a method for rejecting spurious frequency bands in a microstrip filter. The method comprises the steps of operatively connecting a first microstrip resonator to a first feed point, operatively connecting a second microstrip resonator to a second feed point, and operatively connecting a third microstrip resonator to the first or second resonator wherein the third resonator is a ½λ resonator. The third resonator may further comprise a plurality of resonators wherein the position thereof with respect to the first or second resonators being a function of a predetermined rejected frequency range. An alternative embodiment may further comprise the steps of operatively connecting one of the plural resonators on one side of the first resonator, and operatively connecting another of the plural resonators on an opposite side of the first resonator. An additional embodiment of the present subject matter may comprise the step of operatively connecting one of the plural resonators to the second resonator and/or operatively connecting one of the plural resonators between the first and second resonators.
A further embodiment of the present subject matter provides a microstrip filter comprising a first microstrip resonator operatively connected to a first feed point, a second microstrip resonator operatively connected to a second feed point, and at least one ½λ resonator operatively connected to the first or second resonator. The position and number of the at least one ½λ resonator are a function of a predetermined rejected frequency range.
An additional embodiment of the present subject matter provides a method for attenuating selected frequency bands in a microstrip filter having a plurality of microstrip resonators. The method comprises the steps of providing a first of the plural resonators operatively connected to a first feed point, providing a second of the plural resonators operatively connected to a second feed point, and operatively connecting a third of the plural resonators to the first or second resonator wherein the third resonator is a ½λ resonator. The third resonator may further comprise a plurality of resonators wherein the position thereof with respect to the first or second resonators being a function of a predetermined rejected frequency range. An alternative embodiment may further comprise the steps of operatively connecting one of the plural resonators on one side of the first resonator, and operatively connecting another of the plural resonators on an opposite side of the first resonator. An additional embodiment of the present subject matter may comprise the step of operatively connecting one of the plural resonators to the second resonator and/or operatively connecting one of the plural resonators between the first and second resonators.
These embodiments and many other objects and advantages thereof will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the embodiments.